Electrochemical gas sensors are widely used for sensing a variety of different gases. Although the specific design features of these sensors can vary widely based on the electrochemical reactions of the gas species being sensed, the environments in which the sensors are used, and other factors, the sensors generally share common features, such as having two electrodes (an anode and a cathode) separated by an electrolyte. One type of sensor in wide use today is the carbon monoxide sensor. Carbon monoxide sensors generally utilize an anode as a sensing electrode (exposed to air being tested for the presence of CO) and a cathode as a sealed reference electrode that is exposed to clean air, separated by an electrolyte. The reaction that takes place at the sensing electrode (anode) is set forth as CO+H2O→CO2+2H++2e−. The protons liberated by the reaction taking place at the anode are transferred through the electrolyte to the cathode, where they participate in the reaction ½O2+2H++2e−→H2O. The electrons liberated at the anode are conducted to the cathode through a monitored circuit that measures current and/or voltage, with the current/voltage in this circuit being proportional to the concentration of CO in the gas being tested. As can be readily seen by combining these chemical equations, the net reaction of the electrode assembly results in no net production or consumption of water or electrolyte components.
Some designs of carbon monoxide sensors utilized liquid electrolytes such as aqueous sulfuric acid. Such electrolytes, however, are subject to leakage of electrolyte from the sensor assembly, which can expose the surrounding environment to corrosive chemicals, as well as result in degraded performance or failure of the sensor due to electrolyte dry-out. Evaporation of water from such aqueous liquid electrolytes can also result in degraded performance or failure of the sensor. Sensors with such liquid electrolytes can often be utilized only in limited operating environments in terms of temperature and humidity, and also have to include complex design features to isolate the liquid electrolyte from the outside environment. Other designs of carbon monoxide sensors (see, e.g., U.S. Pat. No. 5,573,648) propose using solid electrolyte like the ionomer Nafion®, manufactured by the E.I. du Pont de Nemours and Company. An electrode assembly with such a solid electrolytes is known as a membrane electrode assembly (“MEA”). Such ionomeric electrolytes require the presence of water vapor in order to provide the desired electrolyte performance for gas sensors, and sensor designs that use such ionomeric solid electrolytes typically must have a water reservoir integrated with the sensor in order to maintain humidity levels in the ionomeric solid electrolyte (see, e.g., U.S. Pat. Nos. 6,200,443 and/or 6,948,352). The necessity of a water reservoir adds cost, size, and complexity to the overall sensor design, as well as providing a failure mode for the sensor if the reservoir seal is compromised. Also, since a gas sensor cannot be completely sealed since it must be open at least to the gas being tested, the water reservoir is subject to evaporation and thus has a finite life, which can be further shortened if the sensor is operated in dry and warm environments.
In view of the sometimes demanding requirements for gas sensor electrolytes, various alternatives have been used or proposed. However, new alternatives are always well-received that may be more appropriate for or function better in certain environments, offer better cost, or enable beneficial modifications to the overall sensor design.